Doppler correction technique

ABSTRACT

Method and means for complete correction of Doppler shift in a transmitted spectrum of individual frequencies, or tones, constituting a multitone, coded pulse system in which the lowest (reference) and highest frequencies and the bandwidth are known and enclose an information band of frequencies. The upper and lower half-bands of the received signal are filtered out and mixed to provide the difference frequencies, one of which is a unique frequency corresponding to the spectrum bandwidth. This is filtered out and multiplied by a fixed factor to provide the zero-Doppler reference frequency. The latter is mixed with the information band, which has been filtered from the input signal, and the result is a signal which comprises the information band Doppler-corrected for the reference frequency. The signal corresponding to the spectrum bandwidth is also sent through means which senses the amount of Doppler-shift it has undergone, selects an inverse multiplying factor in accordance with this amount of shift and multiplies the reference-frequency-corrected information band, thereby correcting it for the Doppler-stretch the received information band has undergone. The information band is now completely Doppler-corrected and is divided by a zeroDoppler correction factor to correct the spectral spread introduced by the previous multiplication by said inverse multiplying factor.

United States Patent 3,325,736 6/1967 Waetjen Inventor Edmund C. GannonWaterford, Conn.

Appl. No. 868,369

Filed Oct. 22, i969 Patented July 13, I971 Assignee The United States ofAmerica as represented by the Secretary of the Navy DOPPLER CORRECTIONTECHNIQUE References Cited UNITED STATES PATENTS L808 9/1967 Levin etal340/5 Primary Examiner- Richard A. Farley Attorneys-Richard S. Sciascia,Louis B. Applebaum and Philip Schneider ABSTRACT: Method and means forcomplete correction of Doppler shift in a transmitted spectrum ofindividual frequend es, or tones, constituting a multitone, coded pulsesystem in which the lowest (reference) and highest frequencies and thebandwidth are known and enclose an information band of frequencies. Theupper and lower half-bands of the received signal are filtered out andmixed to provide the difference frequencies, one of which is a uniquefrequency corresponding to the spectrum bandwidth. This is filtered outand multiplied by a fixed factor to provide the zero-Doppler referencefrequency. The latter is mixed with the information band, which has beenfiltered from the input signal, and the result is a signal whichcomprises the information band Doppler-corrected for the referencefrequency. The signal corresponding to the spectrum bandwidth is alsosent through means which senses the amount of Doppler-shift it hasundergone, selects an inverse multiplying factor in accordance with thisamount of shift and multiplies the reference-frequency-correctedinformation band, thereby correcting it for the Doppler-stretch thereceived information band has undergone. The information band is nowcompletely Dopplercorrected and is divided by a zero-Doppler correctionfactor to correct the spectral spread introduced by the previousmultiplication by said inverse multiplying factor.

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This invention relates to a method and means for correcting Dopplershift in coded-pulse communications systems and especially in underseasacoustic coded-pulse communications systems.

Much time and effort have been spent in attempting to developlong-range, underseas, acoustic communications systems. Doppler shiftsin these systems have hitherto been corrected by performing a modulationto shift a band of received frequencies back to a near-zero Dopplercondition. This shift is perfect at only one frequency while all otherfrequencies in the band have proportional errors. Thus, all frequenciesin the band can be measured only to the tolerance governed by the errorremaining after the Doppler correction has been made. For one frequency,the error is zero; for all other frequencies, the error increasesproportionally as the measured frequency moves away from the correctedfrequency. When the measured frequency or frequencies reach a pointwhere the error becomes intolerable, a second point of correction in thereceived spectrum becomes necessary. As one proceeds further along thespectrum, additional corrections become necessary.

This process of corrections has several undesirable effects: (1) theresolution is limited; (2) the use of guard bands between theindependently corrected frequency bands is required causing excessiveoverall bandwidth (usually at a premium); (3) the signal-to-noise ratiois generally reduced because of the necessity of using wider bandwidthfilters to detect frequencies not completely Doppler-corrected, therebyreducing useable range.

An object of this invention is to provide a method and means forcorrecting Doppler shift in communications systems, especially insystems which transmit simultaneous pulses of energy at discretefrequencies over a given frequency band.

The objects and advantages of the present invention are accomplished, ina system in which the zero-Doppler reference frequency and bandwidth areknown factors, by a first correction of the received spectrum throughwhich the Dopplershifted reference frequency is returned to itszero-Doppler value and by a second correction through which theremainder of the transmitted spectrum is multiplied by an inverse factorwhich compensates for the Doppler-induced stretch or compression ofbandwidth and returns it to its original zero-Doppler state.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings wherein:

FIG. I is a schematic diagram showing the basic transmitted frequencyspectrum of the communication system;

FIG. 2 is a block diagram of an embodiment of the invention;

FIG. 3 is a schematic diagram showing, for the zero-Doppler case, thesignal spectrum as it exists at various points in the circuit shown inH6. 2;

FIG. 4 is a schematic diagram showing, for the Doppler-shift case, thesignal spectrum as it exists at various points in the circuit shown inFIG. 2; and

FIG. 5 is a schematic block diagram showing a possible embodiment forthe variable-factor selector mean! and the variable-factorfrequency-multiplying means.

The communication system with respect to which the present inventionwill be described is an underwater acoustic system in which theinformation is presented by a series of simultaneously transmittedtones, each of a different frequency The tones are transmitted for apredetermined period and,

after an interval, another set of tones is transmitted. Each set ofsimultaneously transmitted tones may represent a letter of the alphabet,for example.

The number of tones which may be transmitted is predetermined; that is afixed number of frequency slots (which may be about 0.5 Hz. wide, forexample) exist in the information band of the basic frequency spectrum(see FIG. I) of the transmitter. The frequencies, or tones. which fitinto these slots may be transmitted in any combination-thus, forexample, if there are eight frequency slots, one transmission mightconsist of tones in slots l, 3 and 8 and the next might consist of tonesin slots 2, 3 and 6. This system can be called a multitone, coded pulsesystem. (Strictly speaking, the tones which are transmitted in theunderseas medium are too long in duration to be pulses. However, theywill be designated "pulses" herein.)

Reception of such signals is accomplished by a receiver which employs aset of adjacent narrowband filters to filter out the particular toneswhich were transmitted. However, if the transmitting platform andreceiving platform are moving rela' tive to each other, the Dopplereffect introduces frequency shifts in the received tones, which may besufficient to cause some or all of the tones to be processed through thewrong filters. The receiver then prints out a letter which is differentfrom the one which was transmitted. Thus, it is necessary either toincrease the bandwidth of the filters or to correct the Doppler shift.Since the underseas medium and the electroacoustic equipment nowavailable impose bandwidth limitations, the ability to correct Dopplershift becomes very important.

The principle of this invention is based on the premise that the Dopplershift at any frequency can be considered linear with velocity. Thisassumption is valid for underwater acoustics, since present platformspeeds generally involve a relative velocity of no more than 60 knotsand usually involve considerably smaller velocities in order to maintaina reasonable receiver self-noise level.

To implement the principle of the invention, the receiver must know twothings about the received signal:

1. The zero-Doppler reference frequency, f,;

2. The zero-Doppler bandwidth, or frequency difference,

Thus, in addition to the information band the transmitted signalincludes two other tonesthe reference frequency, f,, which is lower thanthe information tones, and a zero-Doppler, spectrum upper-limitfrequency, f,,, which is higher than the information tones. The spectrumbandwidth, or frequency difference, Af, is then known and equal tof,,f,. The mechanization of this format is simplified if the referencefrequency is some integral multiple of the frequency difference, sothatf,=M(Af) where M is a known integral or fixed factor, which can becalled the reference-frequency factor. Making the reference frequency,f,, a known multiple of the frequency difference, Af, simplifies theproblem of internally generating a reference locked to that in thewater. The internal generation of a reference is necessary in order toavoid effects of amplitude variation in the modulators which use thisreference.

The transmitted spectrum is considered to be divided into two halves,the lower half-band and the upper half-band. Referring to FIGS. 2 and 3,the invention will first be explained with respect to a spectrum whichhas no Doppler shift. The input signalis fed into filter means 10 and 12which separate the input signal spectrum into a lower half-band and anupper half-band@ These half-band signals are fed into first modulatormeans 14 which provides an output@ comprising the difference frequenciesof its input signals and@.Note that the highest frequency obtained is adiscrete and unique frequency corresponding to the bandwidth of thetransmitted spectrum, A].

The output of the first modulator means 14 is coupled into a firstfilter means 16 which may comprise a band-pass filter having a centerfrequency equal to A] and a bandwidth equal,

on either side of the center frequency. to the maximum Dppler shift tobe expected in view of the maximum possible relalive platform velocity.The output @of the first filter means 16 is then a frequency Al.

The outpul of the first filter means to is fed to a fixed-fad torfrequency-multiplying means 18 which has a fixed, predeterminedmultiplying factor, M, (the reference-frequem cy factor) in accordancewith the formula, M(Af)#,. Thus, the output@ of this stage is thereference frequency, f, as received.

The input signalis also coupled to the information-band filter means 24which filters the signal to provide an output@ comprising only theinformation band frequencies, f, to f,. This signal@ and the referencefrequency, 1}, from the fixedfactor frequency-multiplying means l8 arecombined in a second modulator means 26. The lower sideband is thenselected by second filter means 28, providing in effect a downshiftingof the information band so that it now starts from a frequency f,f,.,ends at a frequency f .,f,. The frequency f, in the input spectrum at@,when combined with the signalin the second modulator means 26, becomeszero Hertz in waveform (D Y which signifies I00 percent Dopplercorrection.

The remaining components of the circuit will be explained with theassumption that an up-Doppler shift has occurred during the transmissionof the signal through the medium. Referring to FIG. 4, it is apparentthat the Doppler effect shifts the frequencies upward and stretches themout. Looking at waveforms,@and@,the zero-Doppler location of thereference frequency, f,., has been shifted to a new location,f,'.Thestretch of the individual frequencies is apparent by comparing thelengths of the information band in FIGS. 3 and 4, or thebandwidths-thus, (f,'f,') (f,f,) and A1" Af.

The output@ of the first filter means 16 is now a frequency, or tone,equal to A), the Doppler-shifted bandwidth. When multiplied by the fixedreference-frequency multiplication factor, M, a frequency, f,', theDoppler-shifted reference frequency is obtained at point Modulating theDoppler-shifted reference frequency, f,', with the Doppler-shiftedinformation band@ in the second modulator means 26 and filtering theoutput for the lower sideband brings the information band down infrequency so that its starting frequency is (f,'f,') and its upper-limitfrequency is (f,'f,'). It should be noted that, at this point, thesignal is 100 percent Doppler corrected for the reference frequency,f,that is, if 1", were included in waveform@in FIG. 4 and f, wereincluded in waveform@ in FIG. 3, the modulation process would bringf,.-f,' and f,f, to the same point (the zero point) on the relativefrequency axis in both figures. Stating this another way, thereinsertion of the reference frequency and its modulation with theinformation band frequencies permits the reference frequency to be 100percent Doppler-corrected in all further manipulations of the signal.

The correction of the stretch in the bandwidth is accomplished byfeeding the outputof the first filter means 16 (which is a frequencyequal to the Doppler-shifted bandwidth, Al) to the Doppler shift-sensingfilter means 20. This filter means 20 comprises a bank or set, ofnarrow-band filters completely covering the spectrum of frequencies inwhich M can occur in view of the relative platform velocities for whichthe system is designed. An output is obtained from one of the narrowbandfilters in this set and is coupled to a frequencymultiplication-factorselection means 22.

In the design of the system, a predetermined value, m, is selected as amultiplication factor for the zero-Doppler case. If there is no Dopplershift and the zero-Doppler frequency, Mia sensed by the Dopplershift-sensing means 20 and fed to the variable-factor selector means 22,this zero-Doppler multiplication factor, m, is selected by the selectormeans 22 and fed to the variable-factor frequency-multiplying means 30.The output of the latter is seen (FIG. 3] to be a spread spectrumoccupying the bandwidth from m(f,-f,) to m(f,f,). Division of thisspread spectrum in division means 32, which has a fixed division factor,mfthe zero Doppler multiplication factor), contracts the spread spectrumto its normal bandwidth fl fr) t0 U a Ll This multiplication anddivision is useless in the zero-Doppler case but very useful in aDoppler-shifted case. The multiplication factor, m, which may be calledthe spectrum stretch-correction factor, has a value which is determinedby the equation m,(Aj' )=m(Af). The only unknown in this equation is m,.it is evident that m(Aj) is fixed value once the system design is set.Therefore, m, and Af' vary inversely with each other. If there is anup-Doppler-shift, m, is proportionately decreased below the value of m;if there is a down- Doppler-shift, m, is proportionately increased abovethe value of m. The value of m is thus determined by the value found forA1 and compensates for the difference in values between theDoppler-shifted bandwidth, A1", and the zero-Doppler bandwidth, Af, sothat the equation, m,(Af)=m(Afl, holds true.Thus, the outputof thevariable-factor frequency-multiplying means 30 has the same frequencywidth as the zerodoppler-shifted case and the spectrum stretch caused bythe Doppler-effect has been corrected, although a fixed and invariantspectral spread is now introduced.

The completely corrected, but spread signal is then coupled to adivision means 32 which divides the signal by the zero-Dopplercorrection factor, m thereby eliminating the spectrum spread andproviding the information band frequencies and bandwidth that wasgenerated by the transmitter.

The variable-factor frequency-multiplying means 30 may comprise adelay-line time compressor (DELTIC). Its multiplication factor is verystable and can be varied by a simple lengthening or shortening of theline using shift registers or using a tapped line while simultaneouslyreadjusting the sample rate appropriately.

The division means 32 may comprise a simple array of bistablemultivibrators (flip-flops) arranged in a count-down circuit.

The variable-factor selector means 22 may comprise means for controllingthe sample period and delay-line length of the DELTIC which factors, inturn, control the multiplying factor.

Thus, FIG. 5 shows one way of implementing the variablefactor selectormeans and frequency-multiplying means. Assuming five filters in theDoppler shift-sensing filter means 20, five output signal lines, m mm,,, m and m are fed to a switching matrix means 34. The five outputlines (corresponding to the input signals) of the switching matrix means34 are fed to separate AND gates 36 in the five output lines of a samplecontrol means 46 which includes a sample pulse generator, each linebeing the output channel for a different sampling rate. The sample pulsegenerator may include a delay line with a set of serial shift registersand a recirculation line, for ample.

The switching matrix means output lines are also connected each to adifferent one of five AND gates 38 associated with the shift registers40 of the DELTIC employed as the variablefactor frequency-multiplyingmeans 30. These shift registers 40 are separated into five groupsconnected in serial fashion and functioning as taps on the delay line 42so that the combination is a tapped delay line.

Thus, a signal, e.g., m from the Doppler shift-sensing filter means 20is passed through the switching matrix means 34 to the proper samplecontrol AND gate 36 thereby opening it and selecting a sample pulse ratefor the pulses to be fed through the DELTIC. At the same time, the msignal is fed to the proper shift-register gate 38, thereby opening itand selecting the proper delay time. The sampling rate (or sampleperiod) and delay time selected are the ones which permit the DELTIC tocompensate by the correct amount of signal compression or expansion (thecorrect multiplication factor) for the amount of Doppler shift inherentin the m signal.

The switching matrix means 34 in addition to its switching ability hasan information storage capacity which remembers the last Doppler-shiftsignal that has been fed to it and continues to supply this signal incase the input signal is subject to fading, a common occurrence inoceanic sound propagation.

A circuit which is not essential to the concept of the system but isincluded because of practical considerations is the automatic startmeans 44. During the initial turn-on or after the DELTlC has been dumpedby one of the protective circuits (not shown), it is necessary to clockone pulse into the sample generator to get it started. Basically, thisis accomplished by a countdown mechanism synchronized to the clockfrequency. The countdown mechanism has a count longer than the longestdelay in the sample generator. When the count reaches the end, a pulseis clocked into the sample generator. If the sample is lost or dumped onpurpose and the sample generator normalized, the countdown will againproceed to the end of the countdown mechanism and a new pulse will beclocked into the sample generator.

The automatic start function may, for example, be accomplished by fourdecade counters, two synchronizing circuits, and a gate. The decadecounters wired as dividers give a total division of 10,000. The divideris driven by the clocking frequency and is tapped at the output and atthe divide-by- 1000 point. The output from each tap goes to a pair of ACflipeflops. Each pair is wired as a synchronizer to reestablish theoutput pulse in phase with the clocking frequency. This is done tooffset any propagation delay built up by the countdown mechanism. Theoutputs of both synehroniaer circuits is fed to an AND gate in theserial memory of the sample generator. Ten pulses are fed into the gatefrom the divlde-by-IOOO tap for every pulse from the divide-by-l0,000tap. The division ratios are for a particular embodiment and aredictated by the design criteria of that embodiment. If the designcriteria are changed, the division ratios can be altered accordingly.However, the basic operation would be the same.

Reset of the countdown mechanism is accomplished by having therecirculating pulse in the sample generator trigger the common resetline on all of the countdown decades once every recirculation. If apulse somehow gets lost, the countdown proceeds beyond the reset periodand reclocks a pulse into the sample generator.

lclaim:

l. A method for correcting Doppler-shift effects in a transmitted signalspectrum consisting of plurality of tones of discrete frequencyincluding a zero-Doppler reference frequency, a zero-Doppler upper-limitfrequency and a variable set of information band frequencies enclosedtherebetween, the enclosing frequencies and the frequency differencebetween them, or bandwidth, being known, said reference frequency beingan integral number (M) times the bandwidth, said method comprising thesteps of:

separating the received signal into an upper and lower halfband;

mixing the half-band signals together to obtain the differencefrequencies, one of which is a frequency which has the same value as theDoppler-shifted bandwidth; filtering out the Doppler-shifted bandwidthfrequency; multiplying the bandwidth frequency by said integral number,M, to obtain the Doppler-shifted reference frequency; sensing the amountof Doppler-shift in the bandwidth frequency and obtaining a variablemultiplying factor which is in inverse ratio to the amount ofDoppler-shift;

filtering out of the received signal the Dopplenshifted information bandfrequencies;

mixing said information band frequencies with said Doppler-shiftedreference frequency and filtering the resultant signal to obtain a newset of information band frequencies Doppler-corrected for the referencefrequen' cy; and

multiplying the new set of information band frequencies by said variablemultiplying factor to obtain a set of information band frequencies whichis completely Doppler-corrected but has a spectrum spread.

2. A method as in claim I, further including the step of:

dividing by said zero-Doppler multiplication factor said completelycorrected set of information band frequencies to correct the spectrumspread.

3. A method for correcting Doppler-shift effects in a transmitted signalspectrum consisting of a plurality of tones of discrete frequencyincluding a zero-Doppler reference frequency (f a zero-Dopplerupper-limit frequency (11,) and a variable set of information bandfrequencies enclosed therebetween, the enclosing frequencies being knownand therefore the zero- Doppler bandwidth, AH,,-f,, being known, saidreference frequency being a known integral number (M) times thebandwidth, namely, f M (Af), said method comprising the steps of:

separating the received signal into an upper and a lower half-band;

mixing the half-band signals together to obtain the differencefrequencies, one of which is a tone having a frequency, Aj, the samevalue as the Doppler-shifted bandwidth; filtering out theDoppler-shifted bandwidth frequency; multiplying the bandwidth frequencyby said integral number, M, to obtain the Doppler-shifted referencefrequency. 1,;

sensing the amount of Doppler-shift in the bandwidth frequency andobtaining a variable multiplying factor, m, which is in inverse ratio tothe amount of Doppler-shift and is in accordance with the formula in,(Af ==m (Af), where m is a preselected zero-Doppler multiplicationfactor;

filtering out of the original received signal the Doppler shiftedinformation band frequencies; mixing said information band frequencieswith said reference frequency and filtering the resultant signal toobtain a new set of information band frequencies Doppler-corrected forthe reference frequency; and

multiplying the new set of information band frequencies by said variablemultiplying factor, m, to obtain a set of infonnation band frequencieswhich is completely Dopplercorrected but has a spectrum spread.

4. A method as in claim 3, further including the step of:

dividing by said zero-Doppler multiplication factor, m, said completelycorrected set of information band frequencies to correct the spectrumspread.

5. A means for correcting Doppler-shift effects in a transmitted signalspectrum consisting of a plurality of tones of discrete frequencyincluding a zero-Doppler reference frequency, a zero-Doppler upper-limitfrequency and a variable set of information band frequencies enclosedtherebetween, the enclosing frequencies and the frequency differencebetween them, or bandwidth, being known, said reference frequency beingan integral number (M) times the bandwidth, said means comprising thesteps of:

means for separating the received signal into an upper and a lowerhalf-band;

means for mixing the half-band signals together to obtain thedifi'erence frequencies, one of which is a frequency which has the samevalue as the Doppler-shifted bandwidth;

means for filtering out the Doppler-shifted bandwidth frequency, thebandwidth of said filtering means being approximately equal to plus orminus the maximum expected Doppler shift of the transmitted singal szero-Doppler bandwidth and the center frequency being equal to saidzero-Doppler bandwidth;

means for multiplying the bandwidth frequency by said integral number,M, to obtain the Doppler-shifted reference frequency;

means for sensing the amount of Doppler-shift in the bandwidth frequencyand obtaining a variable multiplying factor which is in inverse ratio tothe amount of Dopplershifl;

means for filtering out of the received signal the Dopplershiftedinformation band frequencies;

means for mixing said information band frequencies with saidDoppler-shifted reference frequency and filtering the resultant signalto obtain a new set of information band frequencies Doppler-correctedfor the reference frequency; and

means for multiplying the new set of information band frequencies bysaid variable multiplying factor to obtain a set of information bandfrequencies which is completely Doppler-corrected but has a spectrumspread.

6. A means as in claim 5, further including:

means for dividing by said zero-Doppler multiplication factor saidcompletely corrected set of information band frequencies to correct thespectrum spread.

7. A means for correcting Doppler-shift effects in a transmitted signalspectrum consisting of a plurality of tones of discrete frequencyincluding a zero-Doppler reference frequency (f,), a zero-Dopplerupper-limit frequency (f,) and a variable set of information bandfrequencies enclosed therebetween, the enclosing frequencies being knownand therefore the zero- Doppler bandwidth, Af=f,,f,, being known, saidreference frequency being a known integral number (M) times thebandwidth, namely, f,.=M( Af), said means comprising the steps of:

means for separating the received signal into an upper and a lowerhalf-band;

means for mixing the half-band signals together to obtain the differencefrequencies, one of which is a tone having a frequency, M, the samevalue as the Doppler-shifted bandwidth;

means for filtering out the Doppler-shifted bandwidth frequency, thecenter frequency of said filtering means being equal to Af, and thebandwidth of said filtering means being equal to maximum expectedDoppler-shift of Af, that is, i f -AD;

means for multiplying the bandwidth frequency by said integral number,M, to obtain the Doppler-shifted reference frequency, f,;

means for sensing the amount of Doppler-shift in the bandwidth frequencyand obtaining a variable multiplying factor, m, which is in inverseratio to the amount of Dopplershift and is in accordance with theformula m, (111' =m (Aj), where m is a preselected zero-Dopplermultiplication factor,

means for filtering out of the original received signal theDoppler-shifted information band frequencies;

means for mixing said information band frequencies with said referencefrequency and filtering the resultant signal to obtain a new set ofinformation band frequencies Doppler-corrected for the referencefrequency; and

means for multiplying the new set of information band frequencies bysaid variable multiplying factor, m to obtain a set of information bandfrequencies which is completely Doppler-corrected but has a spectrumspread.

8. A means as in claim 7, further including:

means for dividing by said zero-Doppler multiplication factor, m, saidcompletely corrected set of information band frequencies to correct thespectrum spread.

1. A method for correcting Doppler-shift effects in a transmitted signalspectrum consisting of plurality of tones of discrete frequencyincluding a zero-Doppler reference frequency, a zero-Doppler upper-limitfrequency and a variable set of information band frequencies enclosedtherebetween, the enclosing frequencies and the frequency differencebetween them, or bandwidth, being known, said reference frequency beingan integral number (M) times the bandwidth, said method comprising thesteps of: separating the received signal into an upper and lowerhalfband; mixing the half-band signals together to obtain the differencefrequencies, one of which is a frequency which has the same value as theDoppler-shifted bandwidth; filtering out the Doppler-shifted bandwidthfrequency; multiplying the bandwidth frequency by said integral number,M, to obtain the Doppler-shifted reference frequency; sensing the amountof Doppler-shift in the bandwidth frequency and obtaining a variablemultiplying factor which is in inverse ratio to the amount ofDoppler-shift; filtering out of the received signal the Doppler-shiftedinformation band frequencies; mixing said information band frequencieswith said Dopplershifted reference frequency and filtering the resultantsignal to obtain a new set of information band frequenciesDopplercorrected for the reference frequency; and multiplying the newset of information band frequencies by said variable multiplying factorto obtain a set of information band frequencies which is completelyDoppler-corrected but has a spectrum spread.
 2. A method as in claim 1,further including the step of: dividing by said zero-Dopplermultiplication factor said completely corrected set of information bandfrequencies to correct the spectrum spread.
 3. A method for correctingDoppler-shift effects in a transmitted signal spectrum consisting of aplurality of tones of discrete frequency including a zero-Dopplerreference frequency (fr), a zero-Doppler upper-limit frequency (fo) anda varIable set of information band frequencies enclosed therebetween,the enclosing frequencies being known and therefore the zero-Dopplerbandwidth, Delta f fo-fr, being known, said reference frequency being aknown integral number (M) times the bandwidth, namely, fr M ( Delta f),said method comprising the steps of: separating the received signal intoan upper and a lower half-band; mixing the half-band signals together toobtain the difference frequencies, one of which is a tone having afrequency, Delta f'', the same value as the Doppler-shifted bandwidth;filtering out the Doppler-shifted bandwidth frequency; multiplying thebandwidth frequency by said integral number, M, to obtain theDoppler-shifted reference frequency, fr''; sensing the amount ofDoppler-shift in the bandwidth frequency and obtaining a variablemultiplying factor, m, which is in inverse ratio to the amount ofDoppler-shift and is in accordance with the formula m1 ( Delta f'') m (Delta f), where m is a preselected zero-Doppler multiplication factor;filtering out of the original received signal the Doppler-shiftedinformation band frequencies; mixing said information band frequencieswith said reference frequency and filtering the resultant signal toobtain a new set of information band frequencies Doppler-corrected forthe reference frequency; and multiplying the new set of information bandfrequencies by said variable multiplying factor, m1 , to obtain a set ofinformation band frequencies which is completely Doppler-corrected buthas a spectrum spread.
 4. A method as in claim 3, further including thestep of: dividing by said zero-Doppler multiplication factor, m, saidcompletely corrected set of information band frequencies to correct thespectrum spread.
 5. A means for correcting Doppler-shift effects in atransmitted signal spectrum consisting of a plurality of tones ofdiscrete frequency including a zero-Doppler reference frequency, azero-Doppler upper-limit frequency and a variable set of informationband frequencies enclosed therebetween, the enclosing frequencies andthe frequency difference between them, or bandwidth, being known, saidreference frequency being an integral number (M) times the bandwidth,said means comprising the steps of: means for separating the receivedsignal into an upper and a lower half-band; means for mixing thehalf-band signals together to obtain the difference frequencies, one ofwhich is a frequency which has the same value as the Doppler-shiftedbandwidth; means for filtering out the Doppler-shifted bandwidthfrequency, the bandwidth of said filtering means being approximatelyequal to plus or minus the maximum expected Doppler shift of thetransmitted singal''s zero-Doppler bandwidth and the center frequencybeing equal to said zero-Doppler bandwidth; means for multiplying thebandwidth frequency by said integral number, M, to obtain theDoppler-shifted reference frequency; means for sensing the amount ofDoppler-shift in the bandwidth frequency and obtaining a variablemultiplying factor which is in inverse ratio to the amount ofDoppler-shift; means for filtering out of the received signal theDoppler-shifted information band frequencies; means for mixing saidinformation band frequencies with said Doppler-shifted referencefrequency and filtering the resultant signal to obtain a new set ofinformation band frequencies Doppler-corrected for the referencefrequency; and means for multiplying the new set of information bandfrequencies by said variable multiplying factor to obtain a set ofinformation band frequencies which is completely Doppler-corrected buthas a spectrum spread.
 6. A means as in claim 5, further including:means for dividing by said zero-Doppler multiplication factor saiDcompletely corrected set of information band frequencies to correct thespectrum spread.
 7. A means for correcting Doppler-shift effects in atransmitted signal spectrum consisting of a plurality of tones ofdiscrete frequency including a zero-Doppler reference frequency (fr), azero-Doppler upper-limit frequency (fo) and a variable set ofinformation band frequencies enclosed therebetween, the enclosingfrequencies being known and therefore the zero-Doppler bandwidth, Deltaf fo-fr, being known, said reference frequency being a known integralnumber (M) times the bandwidth, namely, fr M( Delta f), said meanscomprising the steps of: means for separating the received signal intoan upper and a lower half-band; means for mixing the half-band signalstogether to obtain the difference frequencies, one of which is a tonehaving a frequency, Delta f'', the same value as the Doppler-shiftedbandwidth; means for filtering out the Doppler-shifted bandwidthfrequency, the center frequency of said filtering means being equal toDelta f, and the bandwidth of said filtering means being equal to + or -maximum expected Doppler-shift of Delta f, that is, + or - ( Deltaf''max- Delta f); means for multiplying the bandwidth frequency by saidintegral number, M, to obtain the Doppler-shifted reference frequency,fr''; means for sensing the amount of Doppler-shift in the bandwidthfrequency and obtaining a variable multiplying factor, m, which is ininverse ratio to the amount of Doppler-shift and is in accordance withthe formula m1 ( Delta f'') m ( Delta f), where m is a preselectedzero-Doppler multiplication factor; means for filtering out of theoriginal received signal the Doppler-shifted information bandfrequencies; means for mixing said information band frequencies withsaid reference frequency and filtering the resultant signal to obtain anew set of information band frequencies Doppler-corrected for thereference frequency; and means for multiplying the new set ofinformation band frequencies by said variable multiplying factor, m1 ,to obtain a set of information band frequencies which is completelyDoppler-corrected but has a spectrum spread.
 8. A means as in claim 7,further including: means for dividing by said zero-Dopplermultiplication factor, m, said completely corrected set of informationband frequencies to correct the spectrum spread.